The present invention relates generally to a device and method for determining the presence of specific materials in the environment. More particularly, the present invention relates to a spectroscopic refractometer and method for uniquely determining the presence of materials exhibiting resonance characteristics in the presence of specific electromagnetic oscillations.
Refractometers are well known devices for investigating phenomena causing varations in the refractive indices of various elemental and molecular species. Heretofore, the utilization of refractometers has been limited to the evaluation of the dielectric constant of solids, liquids and gases from the measurement of the fractional change in resonant frequencies between cavities containing (1) a highly stabilized reference sample, whose frequency and dielectric constant are fixed, and (2) the test sample itself.
One device known in the prior art operating in the microwave region of the electromagnetic spectrum utilizes two microwave frequency resonant cavities of either the absorption or transmission type excited in parallel by a frequency modulated linear microwave oscillator such as a modulated reflex klystron. One resonant cavity, which may be sealed under vacuum, functions as a frequency reference while the other resonant cavity, with a slightly different resonant frequency, has a test sample inserted therein. The output signal of each separate resonant cavity is applied to separate crystal detectors. The output signals of the detectors are displaced in time from each other, as a result of both the difference in the resonant frequencies of the cavities and the dielectric constant of the materials therein. Next, the displacement time between the output signal of each detector is measured from which the dielectric constant of the test sample may be calculated as a function of the fractional change in time beyond that normally provided by the difference in the resonant frequencies of the resonant cavities. The critical necessity for a linear proportionality relation between displacement time and the frequency difference between the cavities results in a device which is extremely sensitive to temperature, pressure, and detector component linearity and stability.
Another known device utilized to measure small changes in the refractive index of dielectric materials operating in the microwave region of the electromagnetic spectrum comprises two microwave frequency resonant cavities of the reflection type each individually excited by separate stabilized microwave oscillators such as a reflex klystron whose frequency is controlled by the resonant frequency of its associated resonant cavity. One resonant cavity which is sealed and may be enclosed in a vacuum, functions as a frequency reference while the other resonant cavity, with a slightly different resonant frequency, has parallel perforated end plates enabling injection of a test sample. The output signals of the two stabilized oscillators are transmitted through a microwave waveguide of the appropriate geometry which couples the output signals in an offshoot waveguide (known in the art as a hybrid "T" junction or "magic tee") and applied to a crystal detector whose output is appropriately displayed. Since the output of the detector is a heterodyne beat frequency proportional to the difference in the resonant cavity controlled oscillator frequencies, a change in the dielectric properties of the test sample which produces a proportional change in the sample cavity oscillator frequency can therefore be calculated from the measurement of a proportional change in the detected heterodyne beat frequency.
All such prior art devices must, of necessity, employ a frequency reference resonant cavity if small changes in the refractive index of dielectric materials are to be accurately measured. Also, all such prior art devices require complex setup and calibration procedures, require technically competent personnel to interpret results, and prove quite costly to construct and operate. Additionally and most importantly, the devices are incapable of the unique identification of specific testing materials since any given dielectric constant might be exhibited by any of several materials individually or in combination with other materials.
In the field of spectroscopy, it is generally known that certain elemental and molecular species exhibit resonant characteristics when excited by an oscillating electromagnetic field of specific frequencies. These resonant characteristics are exhibited only when the frequency of the oscillation is such that the energy the molecule emits or alternatively absorbs corresponds to that necessary to induce a transition between energy levels of the molecule. Further, such transitions occur in at least three exemplary modes -- rotational, vibrational, and electronic -- and may occur throughout the frequencies of the entire electromagnetic spectrum for any given resonant material of interest. Since it is known that each material is characterized by its own unique molecular energy level structure, each material exhibits its own unique combination of transition frequencies and such transition frequency spectra are readily available for a wide variety of materials.
It should be noted that many hazardous environmental materials exhibit the aforesaid resonant characteristics. Thus, a device and method for uniquely determining the presence of resonant materials as herein disclosed could be utilized to determine the presence of anesthetics such as ethyl ether or the more commonly employed halogen containing compounds which can escape within hospital operating rooms, the presence of either the noxious oxides of sulfur (e.g., SO.sub.2 and SO.sub.3) or freons in a plant or in the general outdoor atmosphere, or the presence of chlorocarbons such as vinyl chloride in an industrial plant to name but a few.
We have found that an improved refractometer device, which may be referred to as a spectroscopic refractometer, may, therefore, be utilized to uniquely determine the presence of specific resonant materials. Unlike the prior art refractometers which are utilized for the evaluation of dielectric constants, the spectroscopic refractometer uniquely determines the presence of resonant materials by the measurement of the fractional change in the value of a correlating parameter such as the frequency between two resonant cavities whose frequencies are related by the constant value in the absence of the resonant material of interest and whose frequencies are variably related in the presence of the resonant material of interest. Since the two resonant cavities are related by a constant magnitude of the difference frequency at all times other than in the presence of the resonant material of interest, the spectroscopic refractometer is relatively insensitive to temperature, pressure, and detector component linearity, is relatively simple to set up and calibrate, is relatively inexpensive to construct and operate, and may be utilized and operated by laymen to provide a unique determination of the presence of resonant materials.